For more than decades, researches focused on materials of organic conductive molecules and macromolecules were developing rapidly. By virtue of the maturity of organic conductor, insulator, and semiconductor materials, the application of such organic semiconductor materials in electronic devices and optoelectronic devices, such as organic light-emitting diodes, organic laser, organic memory, solar cells, thin film transistors (TFTs), and so on, have been gradually attracted their potential. Generally, the organic semiconductor-based optoelectronic devices possessing the characteristics of being fabricated as thin film device and by low temperature process can be used in various substrates and large size manufacturing process, that is different from the conventional inorganic semiconductor.                The earliest organic light-emitting device (OLED) was disclosed in 1963 by Pope et al., in which a light-emitting phenomenon was observed while a bias voltage of 1000V is being applied on an anthracene crystal of 1 mm in thickness. However, the operating voltage is too high to be applied in real display devices. Current OLEDs and manufacturing methods of the same were disclosed by C. W. Tang and S. A. VanSlyke of Eastman Kodak Corporation in 1987, that a vacuum evaporation method was used to deposit amorphous organic films sequentially on a glass substrate having a transparent electrode of Indium-Tin-Oxide (ITO), and then the substrate with several layer of organic films deposited thereon is plated with a layer of cathode so as to complete an OLED. Such OLED can largely reduce the operating voltage to below 10V enabling the OLED to be used in real world.        
Conventional organic light emitting devices are all forward-stacked structures consisting of bottom anode layer and top cathode layer, as shown in FIG. 1. The organic light emitting device 1 comprises a substrate 11, an anode layer 12, an organic structure layer 13, and a cathode layer 14 stacked sequentially. When the OLED is applied in fabricating an active-matrix organic light emitting display (AMOLED), the organic light emitting device 1 is coupling to the transistors of active-matrix driving circuit arranged on the substrate 11 (not shown in the figure) via the anode layer 12, and it is preferred to employ an equivalent voltage-controlled current source for driving the circuit of the organic light emitting device 1 that the equivalent voltage-controlled current source usually can be a p-channel transistor, as shown in FIG. 1B. However, in a common transistor, such as a-Si field effect transistor (FET) and poly-Si FET, the electric characteristics of p-channel transistor like carrier mobility are obviously inferior to those of n-channel transistor, and thus there is only n-channel transistor available for the a-Si FET. The inverted OLED including the structure consisting of a bottom cathode layer and a top anode layer is an OLED capable of employing n-channel transistors as an equivalent voltage-controlled current source for driving the circuit of the organic light emitting device. As shown in FIG. 2A, the inverted OLED 2 comprises a substrate 21, a cathode layer 22, an organic structure layer 23, and an anode layer 24 stacked sequentially, wherein n-channel transistors, as shown in FIG. 2B, are used as equivalent voltage-controlled current source for driving the circuit of the organic light emitting device, so that not only the design variability of the active-matrix driving circuit can be increased, but also the efficiency of the AMOLED is raised.
The key factors considered while manufacturing an inverted OLED are the charge transported characteristic and the charge injection capability on the interfaces between device electrode and organic material, and between organic material and another organic material. Since most common organic optoelectronic materials have small electron affinities (EA) (i.e. about or less than 3 ev), some metals of lower work function such as Mg, Ca, Li, and Cs are often chosen to be the cathodes of organic light emitting devices. However, such metals usually have high chemical activity and are easy to deteriorate, that increases the difficulty of process control while the OLED is mass-produced. Besides, the different deposition sequence of metal and organic material will also affect the capability of electron injection on the interface of metal/organic.
V. Bulovic et al. has disclosed a method of using an alloy of Mg and Ag as the bottom cathode of an inverted OLED in 1997. However, the electron injection capability is not good enough, that cause the operating voltage of the OLED is still too high. In addition, to use metal Mg for the bottom cathode will result in the problem of easy deterioration due to its high chemical activity so that such method will affect the device characteristics, causing some integration problems in the following processes for manufacturing organic light emitting displays.
In 2002, X. Zhou et al. and S. R. Forrest et al. published an inverted OLED of p-i-n structure, of which the organic material is doped with metals of high activity and low work function, such as Li and Cs, acting as the n-type dopants for promoting the electrons to be injected from the bottom cathode of the inverted OLED into the organic layer. Such method makes it possible to use the conductive materials of low chemical activity but high stability as the bottom cathode of an inverted OLED. However, the electron injection layer of the cathode of the foregoing OLED is formed by doping an organic material layer with metals of high activity and low work function as n-type dopants, that the above mentioned method still suffer the same difficulty of handling metals of high activity and low work function. In addition, atoms of such metals, i.e. Li and Cs, are easily diffused in the organic material, which will affect the operating of device.
In summary, from the past scientific or technical literatures referring inverted OLEDs, the structures and processes of bottom cathode all include the usage of metals with high chemical activity and low work function, which are easily deteriorated and thus affect the characteristics of the OLED. Moreover, it is still difficult to handle the metals with high chemical activity in the process for manufacturing organic light emitting display.